Concentrations in mol/dm³ (HT only)

This topic focuses on calculating the amount of substance in a solution and how to use this in chemical reactions.

Concentration = quantity/volume

Mass concentration, g/ = mass dissolved per dm3 of solvent

- Concentration (g/) = mass (g)/volume ()

Molar concentration, mol/, number of moles dissolved per of solvent

- Concentration (mol/) = moles (mol) /volume ()

This is often rearranged to calculate the number of moles dissolved in solution:

Moles = concentration × volume

• n = c V

  • n = moles (mol)

  • c = concentration (mol/dm³)

  • V = volume (dm³)

Important conversions:

• to : divide by 1000.

Examples

1. 3.0 g of were dissolved in 100 of water. Give the concentration in g/

Answer:

Volume = 100 /1000 = 0.1

c = g/V = 3.0/0.1 = 30 g/

2. Calculate the number of moles of in 250cm3 of a 0.010 / solution

Answer:

Volume = 250 /1000 = 0.250

n = cV = 0.010 x 0.250 = 2.50 x mol

When calculating amount of substance for reacting solutions, the same principle is applied as for reacting masses

• Write the balanced chemical equation.

• Calculate moles of the known solution (using c × V).

• Use mole ratios to find moles of unknown reactant or product.

• Use moles to calculate the unknown quantity as required.

Worked Example:

What volume of 0.150 mol dm⁻³ H₂SO₄ is required to react with 35.0 cm³ of 0.300 mol dm⁻³ NaOH? Give the answer in cm3.

Equation: H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O

Answer:

Moles of NaOH = 0.300 × (35.0 ÷ 1000) = 0.0105 mol

Ratio NaOH:H2SO4 = 2:1

Moles of H₂SO₄ = 0.0105 ÷ 2 = 0.00525 mol

Volume of H₂SO₄ = n / c = 0.00525 / 0.150 = 0.0350 dm³ = 35.0 cm³